The generation of immunologic diversity is dependent on the presence and differentiation of lymphocytes and accessory cells. The ability of many mammalian species to respond to in vivo challenges with complex antigens varies in a time dependent manner during embryonic and neonatal life. Evidence has been presented in a number of in vivo and in vitro systems which document functional or structural immaturity of B cells, T cells, and macrophages or accessory cells in fetal or neonatal animals. In addition, active, non-specific suppression of adult humoral and cellular immune responses by neonatal immunocytes has been reported. Utilizing two in vitro systems, 1) the in vitro primary antibody response to soluble hapten-protein conjugates, and 2) the one way mixed lymphocyte response, it has been shown that cells from neonatal thymus and spleen are both deficient in these responses and are capable of actively suppressing immune response by cells from adult antimals. Populations of both T and non-T cells have been shown to be capable of mediating this suppression. Further studies to determine how environmental exposure to antigen and the mammalian genome interact in controlling phenotypic expression of immune reactivity throughout development are in progress.